1. Field of the Invention
The present invention relates to a silver halide photographic emulsion.
2. Description of the Related Art
Conventionally, grains whose surface have high silver iodide content are known to be unpreferable as a negative photographic light sensitive material since development progress is significantly delayed. For example, J. Photo. Sci., 24, 198 (1976) describes a core/shell type grain whose shell has silver iodide content of 18 to 36 mol %. JP-A-62-19843 ("JP-A" means unexamined published Japanese patent application) describes a core/shell type color reversal photographic light-sensitive material in which the silver iodide content of a shell is higher than that of the core. This material is a reversal light-sensitive material aimed at increasing the sensitivity and contrast of pushing development by using a phenomenon in which development progress is delayed by grains having a high silver iodide content. Therefore, this color reversal photographic light-sensitive material is not suitable as a negative material. In addition, JP-A-49-90920 or JP-A-49-90921 describes grains in which a core consists of silver bromide, a shell consists of silver iodobromide, and a silver iodide content of the shell is 5, 10, or 15 mol %. However, these grains are used in a direct positive emulsion and therefore unsuitable as a negative emulsion. JP-A-56-78831 discloses a monodisperse grain whose surface has a silver iodide content of 6 to 8 mol %. However, these grains are effective only when they are used together with grains whose surface has a silver iodide content of 3 mol % or less, and only low sensitivity can be obtained by using only the former grains.
JP-A-60-147727 discloses, in its scope of claim, grains having a multilayered structure in which a difference between average silver iodide contents of two adjacent layers is 10 mol % or more and a silver iodide content of an outermost shell is 40 mol % or less, but it describes that a preferable silver iodide content of the outermost shell is 0 to 10 mol %. In addition, all of silver iodide contents of the outermost shells of grains described in the embodiments are 3 mol % or less.
JP-A-58-113927 discloses grains having a high silver iodide content in an outermost shell. However, these grains are tabular grains having an average aspect ratio of 8:1 or more.
JP-A-60-14331 discloses grains having a clear double structure but describes that the grains are silver halide fine crystals in which an outermost shell contains 5 mol % or less of silver iodide.
JP-A-61-245151 or JP A 62-131247 discloses grains having a multi-structure. In each reference, however, a silver iodide content of an outermost shell is lower than those of shells inside the outermost shell. In addition, no example in which the outermost shell has a silver iodide content of 6 mol % or more is described in the embodiments.
JP-B-44-15748 ("JP-B" means examined published Japanese patent application) discloses a photographic silver halide emulsion sensitized by at least two types of different sensitizers, i.e., a noble metal sensitizer and a nonlabile selenium sensitizer.
JP-B-43-13489 discloses a photographic silver halide emulsion sensitized by at least three types of different sensitizers, i.e., a noble metal sensitizer, a nonlabile selenium sensitizer, and a nonlabile sulfur compound.